(CNN) -- Episcopal priests will be allowed to conduct services blessing same-sex relationships under a policy approved Tuesday at the church's national convention in Indianapolis.
The convention's House of Bishops approved the provisional policy 111-41 with three abstentions Monday, clearing it for consideration by the House of Deputies, which approved it Tuesday evening.
The policy was approved in the House of Deputies, following more than an hour of debate, by 78% of the voting lay members and by 76% of clergy.
With the vote, the Episcopal Church becomes the largest U.S. denomination to officially sanction same-sex relationships. The Episcopal Church has about 1.95 million members in the United States, down 16% over the last decade, according to the church.
The service is not considered a marriage ceremony, media affairs representative Nancy Davidge said.
"We have authorized a blessing, and a blessing is different than a marriage," she said. "A blessing is a theological response to a monogamous, committed relationship."
Marriage requires the additional involvement of civil authorities, and many states do not allow gays to marry.
The Episcopal policy calls for a three-year trial run of the blessing service, which is called "The Witnessing and Blessing of a Lifelong Covenant."
It will be accompanied by a review process leading up to the church's next annual convention in Salt Lake City. It's then that church leaders would decide whether to make the policy permanent, church media affairs representative Neva Rae Fox said.
The vote in the House of Deputies followed comments, including from advocates arguing the policy would be a step toward inclusiveness.
Delegate Pete Ross of Michigan said it was time for the church to honor lifelong commitments of people in same-sex relationships.
"The signs outside our church says all are welcome. Do we need an asterisk?"
But the Rev. David Thurlow of South Carolina said the policy advocates a "new theology" of human sexuality that is inconsistent with church canons and doctrine.
The approval means the church is "marching off, not simply out of step, but completely out of line from the faith once delivered to the saints," Thurlow said before the vote.
The policy, which has been in development since 2009, allows local bishops to decide whether to allow the service. It also includes a provision stating that clergy members who object to same-sex unions cannot be coerced to perform the blessing, or be disciplined for refusing.
As with civil politics, issues involving homosexuality have roiled American churches for years.
In 2003, the Episcopal Church in the United States split over the election of an openly gay priest, Eugene Robinson, as bishop in New Hampshire. And in 2009, the church approved a policy allowing the ordination of homosexual Episcopalians as priests.
Just this summer, the Presbyterian and Methodist churches rejected measures that would have granted formal church recognition to gay relationships.
The United Methodist Church, at its General Conference meeting in Tampa, Florida, upheld the church's position on homosexuality, which excludes gay marriages and same-sex unions. The Methodist body also rejected a proposal saying the church is not of one mind on the issue of homosexuality.
This month in Pittsburgh, the General Assembly of the Presbyterian Church USA decided not to change the church's definition of marriage as being "between a man and a woman." Delegates approved a two-year study of the issue.
The only major U.S. denomination to endorse same-sex marriage across the board is the United Church of Christ, which did so in 2005.
In 2009, the Evangelical Lutheran Church in America allowed member churches to recognize same-sex relationships, but stopped short of creating a churchwide policy or crafting a specific blessing service.
During Monday's debate, Bishop Nathan Baxter of Central Pennsylvania said the policy would allow the church to focus on inclusion while respecting theological differences within the church, according to the Episcopal News Service.
But others said the policy was a bad idea, the news service reported.
"The Christian world is going to understand us as having changed the nature of the sacrament of holy matrimony," the news service quoted Bishop Edward Little of Northern Indiana as saying. "The Christian world will look at that liturgy world and see vows, and exchange of rings, a pronouncement and a blessing and they will understand that to mean the Episcopal Church has endorsed same-sex marriage and changed a basic Christian doctrine. I do not believe that we are free to do that."
Randall Balmer, a Dartmouth University religion professor and an Episcopal priest who supports the change, said he expects little fallout from the policy within the American church. Most of the most conservative Episcopalians who oppose blessing same-sex relationships have probably already left the church, he said.
"In many ways, the church is tracking public sentiment," which is increasingly supportive of same-sex relationships, Balmer said ahead of Tuesday's vote. "The Episcopal Church is merely part of that trend."
He said it's also unlikely to increase tensions with conservative elements of the worldwide Anglican Communion, of which the Episcopal Church is a member.
"I really don't think it will have a major effect. The real divisions already occurred over Gene Robinson's consecration in 2003," he said.
But some conservatives within the communion might try to use the decision to further marginalize the U.S. church, Balmer said.
CNN's Eric Marrapodi and Phil Gast contributed to this report.